The unit of power is
the watt ..(1000 watts = 1 kilowatt). So in units
form
Electric power (watts) = current (amperes) x voltage
(volts),
where 1 watt = 1 ampere x 1 volt.

1. What is the power when a voltage
of 120 V drives a 2-A current through a device?
2. What is the current when a 60-W lamp is connected to 120 V?A3. How much current does a 100-W lamp draw when connected
to 120 V?
4. If part of an electric circuit dissipates energy at 6 W when
it draws a current of 3 A, what voltage is impressed across it?
5. The equation power = energy converted / time rearranged
givesenergy converted = power x time

Power Production

Does it take a lot of water to
light a llight bulb? That depends on its wattage and
how long it glows. In this practice page, you are to calculate
the mass and
volume of water that must fall over a 10-m high dam in order
to keep a 100-W
light bulb glowing for 1 year.

1. First, calculate how many joules are required to keep the
bulb lit for 1 year.

2. What mass of water elevated
10 m has this much PE? Recall that
gravitational PE = mgh:

3. But this assumes 100% efficiency.
A hydroelectric plant is typically 20% efficient. This means
only 1 part in 5 of the PE of the falling water ends up as electricity.
So the mass above must be multiplied by 5 to get the actual amount
of water that must fall to keep the 100-W bulb lit.

4. This is an impressive number
of kilograms! To visualize this amount of wa ter, convert it
to cubic meters. (Recall 1 kg of water occupies 1 liter, and
there are 1000 liters in 1 cubic meter.)

5. For comparison, an Olympic-size
swimming pool holds about 4000 m3 of water. How many such poolfuls of
water are required to keep a 100-W bulb lit for one year?

Does it take a lot of water to
light a light bulb? To light a city full of light bulbs? Now
you have a better idea!

Teachers Notes

Does it take a lot of water to
light a llight bulb? That depends on its wattage and how long
it glows. In this practice page, you are to calculate the mass
and
volume of water that must fall over a 10-m high dam in order
to keep a 100-W light bulb glowing for 1 year.

3. But this assumes 100% efficiency.
A hydroelectric plant is typically 20% efficient. This means
only 1 part in 5 of the PE of the falling water ends up as electricity.
So the mass above must be multiplied by 5 to get the actual amount
of water that must fall to keep the 100-W bulb lit. 5 x 32
100 000 kg = 160 000 000 kg

5. For comparison, an Olympic-size
swimming pool holds about 4000 m3 of water. How many such poolfuls of
water are required to keep a 100-W bulb lit for one year?
Number of poolfuls = 160 000 m3/4000 m3/poolful = 40 poolfuls